Dome-to-pyramid transition induced by alloying of Ge islands on Si(001)

Ge nanocrystalline islands grown at 650 degrees C on Si(001) are square-base pyramids bounded by {105} facets when they are small and transform to multifaceted, octagonal-base "domes" when they exceed a critical volume. This letter presents the first demonstration that the transition between the two island shapes is reversible, showing that the pyramids are stable, rather than kinetically limited transition structures. Alloying between a Ge island and the Si substrate during annealing at 650 degrees C reduces the strain in an initially dome-shaped nanocrystal. Consequently, the island shape can change from a dome back to a pyramid, even though the island size increases substantially. Stress-enhanced diffusion of Si into Ge likely aids the unexpectedly rapid alloying.